I got my Humu when he was only 2" long about a year ago, now he is almost 6"! I would agree that is is most likely the feeding and diet. All the books or articles I have read about triggers says they need to be fed 2-3 times A DAY! That seems rediculous from a "clean tank" perspective, but I can tell you it can be done.
My Humu is in a 90-gallon with a Passer Angel, Lunare Wrasse and Sharpnose Puffer. I have a 30-gallon sump with basic filter pads instead of Bio-balls. I run the EoroReef E6-1 skimmer and have over 150lbs of LR. I restock the tank monthly with crabs to help clean and provide a live "hunting" food source for the boys to wear down their teeth.
I feed twice a day from a menu of mysis, krill, shrimp, clams, squid, silversides (large and small), angel formula, trigger formula, formula VHP, Prime Reef, Spirulina, beef heart and plankton. In addition, every other week I put in seaweed (which ALL eat) and every month I pick up three fresh raw longneck or cherrystone clams or mussles from the grocery store.
I run zero nitrates on the tank and my fish are happy and healthy. A couple battles have yeilded some cuts and bites every now and then, but they heal and disappear within 3 days.
Hope this was of some help. Good Luck!